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the question is asking this: enter image description here

the correct answers are as follows, Vlimit = 0 V, Vdiode = 5 V

I don't get why it ended up like that. My intuition says that, "fails to open" means not functioning, and so no forward current flowing in the diode, in that case, the voltage across the resistor is 10 V, the voltage across the diode is 0 V.

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4 Answers

up vote 7 down vote accepted

No current is right, because your circuit is interrupted, and you need a closed circuit to get current flowing.

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And when there's no current through the resistor there won't be a voltage drop across it neither, per Ohm's Law: Voltage = Current x Resistance. So 0 V across the resistor means the full 10 V is across the diode (not 5 V!).

If the diode would fail shorted, there wouldn't be a voltage drop across the diode, and the full 10 V would be across the resistor, giving (again due to Ohm) 10 V/ 1 kΩ = 10 mA.

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hmm, im like confuse, we know theres no current in the resistor right, ohms law says 0v. but when we look at it, the left portion of the resistor is exposed to a higher potential, and the right is of a different one, so a potential must exist? correct me if im wrong – vvavepacket Jul 10 '12 at 15:47
Can't possibly be 5V for the Vdiode. Where did the other 5V go? – Martin Jul 10 '12 at 15:48
@Martin - You don't have to complain about this with me, I didn't say anything about 5V! :-) – stevenvh Jul 10 '12 at 15:50
@stevenvh yeah ur right, i just realized im wrong. sorry – vvavepacket Jul 10 '12 at 15:51
1  
@ThePhoton - "1 Megohm or something...". Don't complicate things now! :-) Not in my backyard! ;-) – stevenvh Jul 10 '12 at 16:41
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up vote 6 down vote

"Fails open" is not fail to open. "Fails open" means that it blows up and becomes an open circuit, so in this case there would be the full 10V across its terminals, with 0V across the limit resistor. I don't see how anyone can get 5V.

If it "failed to open" it would presumably remain as a working diode with 0.7V or so across it because it is forward biased.

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up vote 4 down vote

You did not read carefully. The problem states "fails open" (== failed, the failure mode being permanent open), not "failed TO open" as you seem to have read (and which happens to mean the exact opposite!)

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up vote 2 down vote

I think you're wondering the question every student has come to wonder whenever the expected answer doesn't line up with the book's answer:

Is the textbook wrong or is my answer wrong?

The answer here is YES, the textbook answer is wrong. The only way to get 5V across the diode would be if it failed and turned into a 1k resistor. The answers (assuming the book meant what it said when it said the diode fails open are:

Vlimit = 0 V, Vdiode = 10 V

Both sides of the resistor will be at the same potential since no current is flowing through it, and if the diode has failed open, the potential across it will be the potential of the resistor (10V).

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